The present invention relates to vaporization and pressurization of liquid in a capillary material, and relates particularly to formation of a pressurized vapor emission from a non-pressurized liquid source.
Conventional boilers add heat to a reservoir or inflow of liquid to convert the liquid to vapor. To sustain the inflow of liquid in a pressurized boiler system, the liquid must be supplied under at least as much pressure as that of the outgoing vapor. In a typical industrial boiler, the liquid is pumped into the boiler according to the desired vapor pressure. A throttle controls the flow of vapor from the boiler and, correspondingly, the vapor pressure within the boiler. Feed pumps supply water to the boiler according to the vapor pressure to maintain a constant liquid level in the boiler. If the vapor pressure is increased by reducing flow through the throttle, then the pumping pressure is decreased to maintain the level of liquid in the boiler. Usually, the throttle is operatively coupled to the feed pump(s) so that the pumping pressure is automatically adjusted according to the flow through the throttle and, correspondingly, the vapor pressure in the boiler. This mechanism of automatically controlling the performance of the feed pumps is commonly referred to as a servomechanism.
In most liquid fuel vaporization applications, liquid fuel is vaporized, then mixed with air or an oxygen-containing gas, and the vaporized fuel/gas mixture is ignited and burned. The liquid fuel is generally supplied under pressure, and vaporized by mechanical means or heated to vaporization temperatures using an external energy source.
Portable burners and light sources that utilize liquid fuels generate liquid fuel vapor, which is then mixed with air and combusted. Combustion devices that burn fuels that are liquids at atmospheric temperatures and pressures, such as gasoline, diesel fuel and kerosene, generally require the liquid fuel to be pressurized by a pump or other device to provide vaporized fuel under pressure. Fuels such as propane and butane, which are gases at atmospheric pressures but liquids at elevated pressures, can also be used in portable burners and light sources. Storage of these fuels in a liquid form necessitates the use of pressurized fuel canisters that are inconvenient to use and transport, are frequently heavy, may be explosion hazards, and require valves, which are prone to leaking.
The fuel boiler of propane and butane burners is the reservoir or storage tank itself, from which the gases are released under pressure as vapor. When vapor is withdrawn from the fuel reservoir, the pressurized reservoir acts as a boiler, and draws the required heat of vaporization from ambient air outside the tank. These systems have many disadvantages. The vapor pressure of propane inconveniently depends upon ambient temperature, and the vapor pressure is generally higher than that needed for satisfactory combustion in a burner. While butane fuel has an advantageous lower vapor pressure than propane, burners using butane have difficulty producing sufficient vapor pressure at low ambient temperatures. Burners using a mixture of propane and butane fuel provided under pressure in disposable canisters have also been developed. This fuel mixture performs well at high altitudes, but still does not perform well at low ambient temperatures.
A needle valve can be used to control propane vapor at tank pressure to regulate the fuel flow, and thus the heat output, of a burner. Burner control using a needle valve tends to be delicate and sensitive to ambient temperatures. Alternatively, a pressure regulator can be used to generate a constant and less hazardous pressure of propane that is independent of tank temperature. Propane pressure regulators are commonly used in outdoor grills, appliances for recreational vehicles and boats, and domestic propane installations. Unfortunately, regulators are bulky and are seldom practical for application to small-scale portable burner devices.
Despite considerable development efforts and the high market demand for burners for use in stoves, lamps and the like, that operate safely and reliably under a wide variety of ambient temperature, pressure and weather conditions, commercially available combustion devices are generally unsatisfactory.
Wicking systems that use capillary action to convey and vaporize liquid fuels at atmospheric pressure are known for use in liquid fuel burners. U.S. Pat. No. 3,262,290, for example, discloses a liquid fuel burner in which a wick stone is fastened in a fuel storage container and feeds liquid fuel from the fuel reservoir to the burner. In this system, liquid fuel is provided to the wick stone by an absorbent textile wick, and the wick stone is biased against a burner wick.
U.S. Pat. No. 4,365,952 discloses a liquid fuel burner in which liquid fuel is drawn up from a reservoir by a porous member having a fuel receiving section and a fuel evaporation section. Liquid fuel is supplied by capillary action at a rate matching the rate of evaporation of the fuel. Air is supplied to the fuel evaporation section, and liquid fuel is evaporated from the surface at a rate corresponding to the rate of air supply. The gaseous fuel and air is mixed and jetted from a flame section to a burning section. An externally powered heater maintains the porous member of the fuel evaporation section substantially at a constant temperature irrespective of the rate of evaporation of the liquid fuel.
U.S. Pat. No. 4,421,477 discloses a combustion wick comprising a fuel absorption and a fuel gasifying portion designed to reduce the formation and deposition of tar-like substances in the wick. The wick comprises silica-alumina ceramic fibers molded with an organic binder, with part of the wick provided with a coating of an inorganic pigment, silicic anhydride and a surface active agent. The wick may have a capillary bore size of about 1 to 50 microns, with smaller pore size wicks being less prone to accumulation of tar-like substances on the inside.
U.S. Pat. No. 4,465,458 discloses a liquid fuel combustion system in which the liquid fuel is drawn into a porous fiber material or fabric, which is intimately contacted by an externally powered heat generating member to evaporate and vaporize the liquid fuel. Air is introduced to promote vaporization of the liquid fuel and provide an admixed liquid/fuel mixture for burning. Combustion is variable by adjusting the heat input and the air supply.
U.S. Pat. No. 4,318,689 discloses a burner system in which liquid fuel is pumped into a cylindrical chamber having a porous sidewall. As a result of the pressure differential, the liquid fuel penetrates the porous wall to form a film on the external surface of the porous chamber wall. Preheated combustion air entrains and vaporizes the liquid fuel film formed on the external wall of the chamber, and circulates the fuel/air mixture to a combustion chamber. A portion of the hot exhaust or combustion gases may be returned for countercurrent heat exchange to preheat the combustion air.
Although the prior art discloses numerous types of liquid fuel combustion systems, most liquid fuel vaporizers require the application of energy from an external source, such as heat energy, pressure for pressurizing the liquid fuel and/or vapor, or a blower for jetting an air stream to entrain the vaporized fuel for burning. Prior art liquid fuel combustion systems generally provide vaporization of liquid fuels at atmospheric pressures or, if a pressurized vapor stream is desired, either require the fuel supply to be pressurized or pressurize the vapor by external means. Many of the systems are complex and are not suitable for liquid fuel combustion apparatus that are robust, portable or that are suitable for small scale heating or lighting applications.
The vaporization module of the present invention includes a capillary member to convert liquid to vapor in a vaporization zone. The capillary member has low thermal conductivity and small-sized pores that permits liquid to travel by capillary action toward the vaporization zone. Often, the pores of the capillary member are substantially uniform in size. The capillary member may comprise ceramic material. The module also includes an orifice plate that has one or more orifices to permit release of pressurized vapor, e.g. as a pressurized vapor jet. The orifice plate is associated with a sealing member to form an at least partial enclosure of the module so that vapor may accumulate and pressure may be increased within the module. This pressure is sustained by the capillary pressure of the liquid in the capillary member.
In some embodiments, the vaporization module may include a liquid feed member, which may be porous, to provide liquid to the capillary member. Usually the liquid is non-pressurized, e.g. at atmospheric pressure, when introduced to the module. The vaporization module may also include a heat transfer member, which may be porous, to provide heat to the capillary member and in particular, to the vaporization zone. Oftentimes, a thermal gradient is formed between the vaporization zone and the liquid feed member.
Furthermore, some embodiments of the vaporization module include various control mechanisms. For example, a vapor collection space to accumulate vapor and increase pressure may be provided. Such vapor collection space may be formed by the sealing member being positioned away from the capillary member. In addition, the module may have a valve or throttle to regulate the release of vapor. At times, a burner assembly may be provided, for example, in liquid fuel combustion applications to facilitate mixing of gases, e.g. fuel vapors, to form a combustible mixture.
The vaporization module produces pressurized vapor by a method including providing liquid and heat to the vaporization zone. Usually the providing of heat and liquid occurs simultaneously, however, either component may also be provided before the other. At the vaporization zone, the heat is at the liquid vaporization temperature. The resulting vapor is allowed to accumulate in order to build pressure to the desired amount. The pressurized vapor is released from the vaporization module, such as through one or more orifices. Oftentimes, the vapor has a greater pressure than the provided liquid. In some embodiments, the vapor is released with sufficient velocity to mix with air.
The method of making pressurized vapor according to the present invention may be relevant to various fields in which pressurized vapor is desired. In one such field, the released vapor serves as fuel for combustion. In this case, the capillary member may initially acquire heat, such as through an external source, and then the continued source of heat may be from heat of the combustion returned to the module.
An apparatus may incorporate a single module or a plurality of individual vaporization modules, such as in applications requiring more vapor, higher heat or light output than a single module can provide. In addition, modules having different capacities may be arrayed together for use separately or in combination.